This invention relates to stable, aqueous latexes and to methods for their preparation.
Aqueous dispersions of polymers, which are referred to in the art as latexes, are generally known to be useful, both alone and in various formulations, as coatings and impregnants. A wide variety of latexes of differing homopolymeric and copolymeric compositions (such as styrene-butadiene copolymers, acrylic homopolymers and copolymers, vinylidene chloride homopolymers and copolymers, etc.) have been developed having specific chemical and/or mechanical properties for particular end use applications. For example, aqueous interpolymer latexes resulting from the emulsion polymerization of monovinyl aromatic monomers, such as styrene; diolefins, such as butadiene; and monoethylenically unsaturated carboxylic acids, such as acrylic acid; are known to be particularly useful as film-forming binders for pigments in paper coating applications. See, for example, U.S. Pat. Nos. 3,399,080 and 3,404,116. Such emulsion polymerizations optionally employ conventional seeding procedures for optimum control of polymerization and in order to obtain maximum product uniformity (e.g., narrow particle size distribution).
It is desirable to produce diene-containing latexes which exhibit good particle coalescence during film formation. However, the extent of crosslinking reactions of diene-containing copolymers depends upon the monomer to polymer ratio in the reaction locus under isothermal reaction conditions. That is, the rate of crosslinking reactions becomes greater at the lower monomer to polymer ratio (i.e., at high conversions) resulting in highly crosslinked latexes. Consequently, highly converted diene-containing latex particles can exhibit poor particle coalescence during film formation due to highly crosslinked surfaces. This results in deficient water and solvent resistance and inferior mechanical properties of both dry and wet latex films.
Chain transfer agents provide a useful way for controlling the molecular weight and reducing the effect of severe crosslinking upon latex particle coalescence. However, the deformability or elongation and, thus, coalescence of latex films increases with increasing amounts of chain transfer agents only upon sacrificing the tensile strength of the latex film. Therefore, the optimum level of chain transfer agent has to be a compromise between these opposing effects.
In view of all of the previously stated obstacles, it is highly desirable to minimize the extent of crosslinking of diene-containing latexes at the latex surface domain, and to develop latexes that are highly water and solvent resistant and are highly coalescence-capable.